In the printing industry, labels which are carried on an underlying web or carrier substrate usually are spaced apart with gaps between them. Such labels are typically detected using optical sensing devices. These sensors are configured to operate in the transmission ("see-through") mode, detecting the difference in optical density between the backing material (substrate) and the backing plus the label. Alternatively, in the reflecting mode, the sensor detects printed features on the labels.
Recently, transparent materials have become common for labelling applications, as well as for the substrate material. If both the substrate and the label are transparent, optical sensors tend not to be suitable, since it is difficult to detect minute differences in the optical properties of substantially transparent materials. Fluorescent adhesive materials between the label and the carrier have been used occasionally to help differentiate optically between the label and the backing material.
In the past, surface features of a travelling web have been detected utilizing a single nozzle with a pressure sensing device positioned in the air flow path to the nozzle. As the distance between the nozzle outlet and the surface changes, the pressure and the corresponding sensor output signal changes more or less with the reciprocal function of the distance. In this prior approach, the output signal of the sensor is a function of the absolute distance of the nozzle to the surface, as well as the supply pressure. In conventional arrangements of nozzle and sensor, the response time of the system places a limit on the speed at which sheet material with small surface features can be moved past the nozzle.
Another approach has utilized contact rollers pressed against the surface, in conjunction with displacement-sensing devices, to detect the presence of the labels. This method has limitations in speed, vibration interference and spatial resolution.